We propose a Program Project that addresses two crucial questions in vertebrate development: (1) how stable phenotypic differences arise in cell lineages of multicellular embryos, and (2) how migration and localization of different embryonic cell populations and neuronal processes is controlled. We propose to examine two cell populations--the neural crest and primary neurons--in a live vertebrate embryo, the zebrafish, Brachydanio rerio, where development can be observed and characterized directly. The neural crest gives rise to a variety of cellular phenotypes, many with useful markers of terminal differentiation. Since crest cells appear to disperse, localize and differentiate in response to developmental cues encountered very early in development, they are ideal for studying the role of embryonic environments in regulating cellular differentiation and morphogenetic behavior. Primary neurons underlie the earliest embryonic behaviors. They arise early in embryonic development, and are the first neurons to grow axons. The primary motor axons, which are the first axons to appear in the periphery, navigate through the same interstitial environment at the same time as crest cells. We will study these two systems using cellular, molecular and genetic approaches. First, to understand the relative roles of cell lineages and environment in specification of cell fate, we will determine the detailed lineages of individual crest cell precursors and we will analyze the migration and interactions of crest cells with their environment in living embryos. Second, we will use monoclonal antibodies to identify molecules that may determine neuronal specification. Using these antibodies as probes, we will isolate and ultimately characterize the genes that code for these developmentally important molecules. Finally, we propose to characterize genetic and developmental mechanisms that determine cell fates by generating and analyzing mutations that affect neural crest development.